1. Field of the Invention
The invention relates to a sputtering apparatus for depositing a thin film on a substrate surface.
2. Description of the Prior Art
Fin-film deposition on a substrate surface is widely carried out in fabrications of electronic devices such as large-scale integrated circuits (LSI) and liquid crystal displays (LCD). Because of high deposition rates, sputtering apparatuses are often utilized in this thin-film deposition.
FIG. 6 shows a schematic front view of a conventional sputtering apparatus. The sputtering apparatus shown in FIG. 6 comprises a vacuum chamber 1 having a pumping system 11, a cathode 2 provided in vacuum chamber 1, a target 2 which is a component of cathode 2 and is sputtered for deposition, a substrate holder 3 for placing a substrate 30 so as to face to cathode 2 in chamber 1, and gas introduction system 6 for introducing a gas necessary for sputter discharge (hereafter called "discharge gas") into chamber 1. The apparatus shown in FIG. 6 is a magnetron sputtering type. Specifically, cathode 2 is composed mainly of a magnetic assembly 4 and a target 5 provided at the front side of magnetic assembly 4.
The apparatus shown in FIG. 6 is operated as follows. After pumping vacuum chamber 1 by pumping system 11 to a certain pressure, a discharge gas such as argon is introduced by gas introduction system 6. Introducing the discharge gas, a negative direct current (dc) voltage is applied with cathode 2. Sputter discharge is ignited and sustained with the discharge gas, and thereby target 5 is sputtered. Sputtered material of target 5 arrives at substrate 30 to deposit a thin film on the surface of substrate 30.
In sputtering apparatuses as described, a thin film is sometimes deposited overlying on another dissimilar thin. For example, a barrier film is deposited by sputtering for preventing cross-diffusion between a contact wiring layer and an underlayer. A multilayer structure where a titanium nitride film is overlaid on a titanium film is employed for this barrier film.
Process of such dissimilar-films deposition will be described taking the barrier film into an example. In composition of the apparatus shown in FIG. 6, target 2 is made of titanium. First of all, target 2 is sputtered introducing a normal discharge gas such as argon by gas introduction system 6. As a result, titanium film is deposited on substrate 30. Next, nitrogen gas is introduced by an auxiliary gas introduction system 7 connected with gas introduction system 6. Target 2 is sputtered through the sputtering discharge of nitrogen gas. As a result, titanium nitride film is deposited on the nitride film, utilizing reaction of titanium and nitrogen. The structure where the titanium nitride film is overlaid on the titanium film is obtained.
In the process of such dissimilar-films deposition as described, there are problems such as deposition non-uniformity brought by diversity of erosion profiles across a target depending on kinds of discharge gases and precipitation of reaction product on the target surface of a shallowly eroded area. These problems will be described using FIG. 7. FIG. 7 is schematic cross-sectional view for describing problems in conventional sputtering apparatuses.
A cathode configuration and an erosion profile in conventional sputtering apparatuses are shown in FIG. 7. A magnetic assembly 4 is mainly composed of a center magnet 41, an outer magnet 42 surrounding center magnet 41 and yoke 43 interconnecting center magnet 41 and outer magnet 42. As shown in FIG. 7, poles of center magnet 41 and outer magnet 42 at those front surface are different from each other. This is why arch-shaped magnetic lines penetrating through target 5 provided in front of magnetic assembly 4 are applied.
In magnetron discharge, as known, discharge is most intense where directions of electric field and magnetic field make rectangular. In the sputtering apparatus shown in FIG. 6 and FIG. 7, the discharge is most intense at peaks of arch-shaped magnetic lines or near, because the electric field is applied along the thickness direction of target 5. Therefore, generally target 5 is eroded deeply at the region facing to the peaks of magnetic lines and shallowly at center and peripheral regions.
When material of target 5 is titanium and nitrogen gas is introduced into vacuum chamber 1, titanium and nitrogen react at the surface of target 5, inner space of vacuum chamber 1 where sputtered titanium atoms fly, or the surface of substrate 30 which titanium atoms are sticking on. Precipitating titanium nitride dose not remain much in the surface region where target 5 is eroded deeply because precipitating titanium nitride is sputtered out of the surface efficiently. Contrarily, as designated with "500" in FIG. 7, precipitating titanium nitride remains at the surface region where target 5 is eroded shallowly because precipitating titanium nitride 500 is not sputtered out.
Because sticking force of this precipitating titanium nitride with target 5 is week, titanium nitride tends to exfoliate easily when it grows to some amount. When titanium nitride 500 exfoliates, fractions of titanium nitride 500 fly in vacuum chamber 1 to become dust particles. If those particles arrive at the surface of substrate 30, problems such as a local film-thickness defect and film contamination arise. To solve these problems resulting from the exfoliation of precipitating titanium nitride, sputter-cleaning cleaning is carried out during an interval between the titanium nitride depositions. The sputter cleaning is carried out by sputtering the precipitating titanium nitride out of the target surface introducing such an normal discharge gas as argon. Because with conventional sputtering apparatuses this sputter-cleaning needs to be carried out much often, production efficiency of those apparatuses is worsen causing productivity decrease.
When sputtering is carried out switching to a dissimilar gas as in case that titanium film and titanium nitride film are deposited using common target 5, a problem such as deposition non-uniformity arises resulting from that erosion profiles differ depending on kinds of introduced gases. For example, because argon and nitrogen have a different ionization coefficient and a different sputter efficiency, the eorosion profiles differ a little even if the same magnetic assembly 4 is used. In addition, plasma density distributions also differ a little depending on kinds of discharge gases. From these factors, the erosion profiles differs between in case introducing argon and in case introducing nitrogen.
To make an erosion profile uniform, conventional apparatuses revolve magnetic assembly 4 eccentrically from the axis of target 5. However, even by this method, an erosion profile is made uniform only in case introducing a specific gas. This is because the eccentric revolution of magnetic assembly 4 is just composed so that erosion can be made uniform simulating introduction of a specific discharge gas. In other words, because magnetic assembly 4 is composed so that an erosion profile is made uniform when a specific discharge gas is introduced, another erosion profile is not made uniform when another discharge gas is used, which leads to the non-uniform deposition on the surface of substrate 30. It is supposed that this problem can be solved by replacing magnetic assembly 4 according to discharge gases. However, replacement of magnetic assembly 4 costs so much and makes the structure of the apparatus so complicated.